The development of new light sources, for example, LEDs (light emitting diodes), photosemiconductor devices such as semiconductor lasers, and power devices using wide band gap semiconductors has been promoted from the viewpoints of environmental problems and energy saving.
Regarding semiconductors for use in these devices, gallium nitride (GaN) base semiconductors such as GaN, InGaN, AlGaN, and InAlGaN have drawn attention and have been used as layers constituting the devices. For example, LED elements have a structure including a stack of a plurality of thin GaN-base layers. For example, Japanese Patent Laid-Open No 111766/2004 (patent document 1) uses a multilayer structure of a GaN layer and a GaAlN layer. The yield of semiconductor is governed by the efficiency of the formation of and the evenness of thickness of the thin semiconductor layers.
Epitaxial growth is generally used for the manufacture of gallium nitride (GaN) base semiconductor devices. Up to now, sapphire or SiC substrates have been used as epitaxial substrates. These substrates, however, suffer from problems such as high cost (sapphire and SiC). Among others, since the sapphire and SiC substrates are single crystals, difficulties are encountered in increasing the substrate size. Further, in recent years, the growth of the GaN layer using sapphire substrates having a larger size has been desired from the viewpoint of increasing the number of acceptable semiconductor chips obtained per semiconductor substrate.